1. Field of the Invention
The present invention relates to ultrasonic transducer arrays and, more particularly, to intersecting transducer arrays and methods for making the same.
2. Related Art
In a preferred embodiment of the invention discussed below, a biplane phased array transducer device is provided for applications such as medical imaging and/or treatment (therapy) and, more particularly, for use in endocavity ultrasonic imaging probes for trans-rectal and trans-vaginal applications where multisectional views of the examined organ are often desired for diagnosis enhancement.
Currently, there is a large number of different ultrasonic probes designed for many different specific applications, clinical needs and other requirements. Biplane array transducers are representative of a particular group of ultrasonic probes that is especially suitable for use under certain conditions, i.e., where only limited manipulation or movement of the probe is possible. Such probes are, therefore, generally equipped with two separate array transducers disposed in such a manner that the scanning planes thereof intersect in the field of view. This has obvious advantages because the organ being examined is simultaneously separated in two different orientations.
A very early, if not the first, biplane transducer device for imaging applications was disclosed in U.S. Pat. No. 3,881,164 to Kossoff, wherein an apparatus for ultrasonic examination comprising first and second intersecting arrays is described. In this patent, one of the arrays is constructed as the primary transducer whereas the other array is disposed perpendicularly thereon and is complementary to the primary array. The complementary array is interrupted by the passage of the primary array therethrough, and no manufacturing method for the apparatus is disclosed in the patent.
In U.S. Pat. No. 4,570,488 to Miwa et al, there is proposed a method for observing two distinct sections of the object of examination simultaneously, using two perpendicular transducers operated at the same time. In one preferred embodiment, both of the arrays are constructed such that the signal and ground electrodes are formed on the same surface of the piezoelectric member. The intersecting arrays possess an overlapping area which is defined in the elevational dimension of the arrays. Because the signal electrodes cross each other, the overlapping area is of a matrix shape. This matrix area is composed of hundreds of individual small surface elements. Although the design of this transducer array appears to be quite simple, providing electrical interconnections in the matrix area is a difficult task with respect to both device construction and system upgrade because each element in the overlapping area must be addressed.
Another biplane transducer array combining perpendicularly intersecting array electrodes is that disclosed in U.S. Pat. Nos. 4,671,293 and 4,870,867, both to Shaulov. In the '293 patent, a rectangular biplane phased array is described which uses a 1-3 piezoelectric composite as the active component. The major surfaces of the piezoelectric member are plated to form signal electrodes. Thereafter, dicing is performed through the electrodes, and partially into the piezoelectric composite member, to form patterns of perpendicularly oriented electrodes. The electrodes are successively connected to the excitation signal and ground of the system to provide biplane operation of the transducer. The '293 patent discloses that partial dicing (dicing to a depth of 25-90% of the total thickness) in the piezoelectric material is compulsory for achieving acceptable transducer performance. The '867 patent relates to a variation wherein the transducing device is still built with intersecting portions but including protruding branches. Electrode patterns are simultaneously disposed on the top and bottom surfaces of each branch of the transducer to form transducer arrays of an intersected shape. Because the arrays have their electrodes laminated on the opposite surface of the piezoelectric member, no matrix section of electrodes is formed as in the prior art. However, the patent relates only to driving of the transducer array and to the method of dicing the elements of the transducer array, and does not disclose a solution to the problem of, nor a method for, manufacturing the array.
Conventionally, ultrasonic devices are designed based on one or more electrode plated piezoelectric members having a first surface facing the examination medium and a second surface loaded with an attenuating backing member. When the transducer is electrically excited, ultrasonic energy is emitted from both of the opposite surfaces of the piezoelectric member, although only the energy from the front face of transducer is of benefit with respect to producing the desired scanning images. The acoustic energy emitted from the rear surface of the piezoelectric member is cancelled by attenuation or backscattering in the backing member so as to avoid artifacts. In order to enhance the energy transmission coefficient along the front path, the transducer is commonly equipped with a front impedance matching device or impedance matching layers which also perform frequency accentuation filter functions to provide broadening of the bandwidth of the transducer device. As long as the ultrasonic transducer employs heavy acoustic impedance piezoelectric materials (ceramics, composites, single crystals and the like) for producing the ultrasounds, such front path matching layers are required to provide acceptable performance with respect to imaging biologic tissues.
In conventional constructions, as well known in the art, the transducer array is commonly composed of a plurality of diced piezoelectric elements which are maintained in place by means of the backing member disposed on the rear face of the piezoelectric member. An individual acoustic matching device is provided on the front surface of every piezoelectric element, and kerfs (void spaces) between the piezoelectric elements are usually filled with flexible resin to physically isolate one piezoelectric element with respect to its neighbors. The rear electrode of the transducer array is connected to a flexible (flex) printed circuit which connects elements of the transducer array to coaxial cables, optionally via intermediate connectors. In general, the front electrodes of transducer are all shunted together and then connected to the system zero potential. Optionally, an independent overall shield is advantageously provided over the external surface of the transducer device to improve EMI immunity. As long as the array transducer is shaped in 1D configuration, providing a connection to ground in this way is quite convenient and is well suited to both linear and curved linear arrays.
With regard to the intersecting transducer arrays as disclosed in some of the prior art, the transducer comprises at least two intersecting arrays which overlap in the central area of the device. In order to enable each separate array to operate properly, the central area comprises portions of electrodes disposed such that the elemental electrodes or elements can be respectively obtained for the two arrays. To achieve such an electrode mapping, the central electrode is split into regular portions forming a matrix of electrodes (as in U.S. Pat. No. 4,570,488 described above) wherein each element is independently addressed. A multiplexing device is thus required to ensure that the proper elements of the array are addressed in the appropriate sequence during operation. Control of such a multiplexing operation is complex and an appropriate multiplexing device could be more complex to manufacture than a matrix array itself.
Another limiting aspect of an intersecting transducer array construction concerns the acoustic behavior of such a transducer array. It has been widely reported in the literature that several spurious vibration modes occur strongly when the transducer is excited. One of the most common and probably the most important of the spurious modes is created by Lamb (or plate) effects that result from local vibrations of the piezoelectric member. Several species of Lamb waves may simultaneously exist but all differ in velocity and polarization. In general, the lowest order modes of Lamb waves are transversely polarized and propagate along the wave guide (transducer member). The propagation velocity of such modes is governed by the elastic and shear properties of the piezoelectric material. As a result, in a sheet material of limited thickness, such as the piezoelectric member of the transducer or the matching layers, the velocity of Lamb wave may be higher than the compressional sound velocity, thereby favoring the transmission of unwanted acoustic energy in the direction of the interrogation, with a deviation angle corresponding to the ratio between the Lamb wave velocity and the longitudinal velocity in the medium. This spurious acoustic radiation dramatically restricts the angular response of the transducer so that as a result, artifacts can be observed in the resultant images.
In order to combat the effects of Lamb wave propagation, nonhomogeneous inclusions or barriers have been provided in the support member which produce strong attenuation or backscattering of the transverse wave propagation so as to improve the quality of the array performance. One of the most common and efficient methods used here is to cut through or into the material forming the member and then fill the resultant void spaces with a flexible resin or particle filled polymer to provide a physical barrier against coupling waves. Currently, such a through cut method is widely used in PZT D1 array transducer fabrication. Once the transducer is assembled, a dicing operation is performed at least through the piezoelectric thickness to produce individual transducer elements spaced from each other. Optionally, grooving of the backing member can also be provided in a manner to attenuate mass/spring modes that particularly affect phased array constructions. Unfortunately, with respect to an intersecting array construction such as disclosed in the prior art discussed above, the through cutting techniques previously described are impractical for use in manufacturing of such an array because of the particular arrangement of the two intersected arrays.
Because of the problems discussed above, conventional biplane devices having an intersecting configuration are commonly made using a partial cut method so as to preserve the transducer by avoiding a complete separation of the array elements. Further, no crossed shape intersecting array configuration is fully disclosed in the prior art because of the task of connecting all of the elements of the intersecting arrays.
With regard to a suitable method for manufacturing intersecting arrays, even if a through cut operation combined with resin filling of the resultant kerfs might appear to be an ideal solution in certain configurations (i.e., a 1D array) for avoiding Lamb wave propagation, such an approach using a partial cut into the piezoelectric member will not yield a proportional effect on the Lamb modes. In this regard, partial cuts into the piezoelectric member will result in the creation of two sub-structures, one formed by the portion of the member defined by the thickness of the grooves and the other comprising the remaining portion of the member. Each sub-structure can be considered to be a “new” or different member and, therefore, the initial problem is increased because of the combined spurious wave resulting from the two sub-structures. As a result, if the piezoelectric member cannot be through cut and filled with an attenuating material, the best way for making a reliable and efficient transducer imaging device might be to use a 1-3 composite piezoelectric wherein the material structure already comprises vertical thin pillars embedded in a resin matrix. Such ceramic pillars will offer natural obstacles to Lamb wave propagation thereby making the composite an efficient base material for an uncut transducer.
Another factor that significantly limits the use of current biplane array transducers is the need to use dedicated or upgraded hardware to effect the required transducer switching. In this regard, imaging systems are conventionally designed for operation in conjunction with a 1D scanning probe which employs a linear transducer arrangement. The transducers of this arrangement are disposed along the major axis thereof (azimuth) to provide electronic beam forming and focusing. Each probe is electronically coded and is automatically identified by the system when plugged in, and a corresponding beam forming program is then uploaded. The scanning image is displayed as a 2D (planar) representation wherein the vertical dimension represents the penetration of the image while the lateral dimension corresponds to the transducer azimuth geometry associated with the system steering amplitude. Thus, using a biplane array transducer with a 1D conventional imaging system will result in incompatibility and consequently, in incorrectly displayed images.
In order to permit the use of biplane array transducers in conventional imaging systems, some basic conditions must be fulfilled. For example, the existing system is provided with at least two probe connectors wherein the first array of the probe is plugged in to the first connector while the second (intersected) array of the probe is connected to the second connector of the system. With this arrangement, users simply have to select, on the system interface, either the first connector or the second connector in order to provide the desired operation. Both arrays are expected to be identified by the system as known thereto.
Given the aforementioned shortcomings with respect to combating Lamb wave effects and the lack of compatibility of biplane ultrasonic transducers with existing imaging systems, there is obviously a need for both an improvement in transducer performance and with respect to the incorporation of switching features between the crossing arrays of the transducer in order to make the use thereof transparent to the system and to users of the system.